The present invention relates to a low temperature thermal energy storage composition which utilizes the latent heat of phase change to store heat.
Low temperature thermal storage materials are well known in the prior art. Rocks, water and other fluids are often used, however the excessive bulk and weight of the material needed to store a sufficient amount of heat deters one from their use. The use of thermochemical heat storage, wherein the latent heat of a phase change is utilized, permits one to achieve compactness of the heat storage material.
Materials suitable for heat of phase change storage have a number of desirable properties, among which are a phase change in a practical temperature range (usually about 90-200.degree. F.), a high energy density (that is, a high latent heat of phase change per unit volume), and low cost.
One such heat of phase change material is the decahydrate of sodium sulphate, also known as Glauber's Salt or Mirabilite, occurring naturally or produced synthetically. It has the chemical formula Na.sub.2 SO.sub.4.10H.sub.2 O. Glauber's Salt is particularly attractive because it is readily available, inexpensive, and non-toxic, and the storage space required is small when compared to non-latent heat type storage materials. For instance, as related by Mr. F. Lindner in a paper given at the Energy and Politics Forum of the Government of Baden-Wuerttemberg at the University of Stuttgard, May, 1977, to attain an equivalent heat storage capacity, a quantity of rocks 34 times heavier and 27 times larger, or a quantity of water 6.5 times heavier and 11.5 times larger than Glauber's Salt would be needed.
Glauber's Salt is known to melt in the crystal-bound water at a moderate temperature of 90.8.degree. F., storing approximately 108 BTU/lb. as latent heat of phase change. Recrystallization of the melt as it is cooled releases the majority of this stored energy as recoverable heat.
The use of Glauber's Salt as a heat storage material is reported in U.S. Pat. Nos. 2,677,664 and 3,986,969, issued to Telkes.
At least two major problems exist in any attempt to utilize the salt hydrate for heat storage.
Firstly, upon cooling a melt of Glauber's Salt the mixture tends to exhibit supercooling and thus the latent heat of recrystallization is not fully recoverable. Telkes, in U.S. Pat. No. 2,677,667 found that the problem of supercooling could be overcome with the addition of a nucleating agent. Particularly, sodium tetraborate decahydrate (Na.sub.2 B.sub.4 O.sub.7.10H.sub.2 O) has been proven to be effective.
Secondly, Glauber's Salt, on melting, exhibits incongruent melting; that is, two new phases are formed. One such phase is a metastable supersaturated aqueous solution of sodium sulphate, the water of solution being wholely derived from the water of hydration of Glauber's Salt representing 56% of the original mass. The other phase is solid anhydrous sodium sulphate, representing approximately 18% of the original mass of the unmelted Glauber's Salt; this latter phase, having a density of approximately twice that of the solution phase, settles to form a layer on the bottom of the container. On cooling, the sodium sulphate dissolved in the solution phase begins to rehydrate with the water of solution to form Glauber's Salt crystals which, having a higher density than the surrounding solution, settle on top of the layer of anhydrous sodium sulphate, thereby preventing a large fraction of this material from rehydrating with the water of solution upon further cooling. This large fraction is thus removed from further use for heat storage, reducing the heat storage capacity of the system.
One solution to the problem of segregation resulting from incongruent melting has been to apply mechanical mixing to the melted solution and settled or segregated layer of anhydrous sodium sulphate. As developed by Herrick and reported in Business Week, Jan. 16, 1978, the unmelted Glauber's Salt is filled and sealed into a cylindrical container. After melting and during cooling and further cycling, the cylinder is continuously rotated slowly with its axis in the horizontal plane causing the segregated layer of anhydrous sodium sulphate to be lifted and then overturned through the bulk of the solution, whereby substantial rehydration may be encouraged. However, this method suffers from the disadvantage of requiring extra input of mechanical energy derived from an external power source and maintenance of a rotating drive and suspension system.
Another approach reported by D. D. Edie and S. S. Melsheimer in "Sharing the Sun", Volume 8, 1976, Pages 262 to 272 considers providing agitation and turbulence of the anhydrous sodium sulphate phase by circulating an immiscible fluid of lower density than the salt solution from the bottom to the top of the container. The bubbling action of the immiscible fluid flowing up through the bulk serves to stir up the anhydrous layer, thus exposing it to rehydration during cooling. In this approach, an additional energy expense in the form of fluid pumping is required to accomplish the objective of rehydrating the segregated anhydrous salt.
A better approach to this problem appears to be the provision of a type of lattice network or dispersant to keep the anhydrous salt suspended or trapped within the bulk of the solution. Telkes, in U.S. Pat. No. 3,986,969, has taught suspending the salt hydrate in a thixotropic gel, as provided by an aqueous solution of attapulgite clay.
The present applicant has investigated this clay-salt mixture and has found, after subjecting it to a number of heat-cool cycles, that the thixotropic gel can break down, allowing a portion of the anhydrous salt to settle out of solution.